At present, a flat-panel display as an embodiment of a color display apparatus has widely been popularized as various monitors for a personal computer and the like and as a display apparatus for a cellular phone, and so on. In the future, the flat-panel display is expected to follow popularization more and more, such as development in use for big-screen television.
A most popular flat-panel display is a liquid crystal display apparatus. As a color display method for the liquid crystal display apparatus, one called a micro-color filter method has been used widely.
The micro-color filter method effects full-color display by constituting one unit pixel with at least three pixels and providing the three pixels with color filters of three primary colors of red (R), green (G), and blue (B), respectively, thus having an advantage of readily realizing a high color-reproducing performance.
On the other hand, as a disadvantage of the micro-color filter method, a transmittance is ⅓ of a monochromatic display method, so that a light utilization efficiency is low.
This low light utilization efficiency leads to a high power consumption of a back light or a front light in a transmission-type liquid crystal display apparatus having the back light or a reflection-type liquid crystal display apparatus having the front light.
On the other hand, a display device having complementary color filters of yellow (Y), cyan (C), and magenta (M), not the RGB color filters has been proposed (“Preprint of the Japanese Liquid Crystal Society Annual Meetings 1998”, pp. 324˜). However, it was impossible to display high-purity primary colors only by applying the YMC color filters to a monochromatic display device simply based on a conventional concept.
Incidentally, in recent years, as a liquid crystal display device used in a liquid crystal display apparatus, a transflective (semi-transmission)-type liquid crystal display device having alight reflective area as a part of display area and a light transmissive area as a part of display area has been proposed in U.S. Pat. No. 6,466,280. Such a transflective-type liquid crystal display device has been widely used in a cellular phone, a mobile data terminal, etc. Incidentally, a portable electronic apparatus is used outdoors in many cases, so that the apparatus is required that it ensures not only a sufficient viewability in extremely bright extraneous light but also a high contrast and a good color reproducibility even in a dark room.
Further, there have been recently reported some display devices, as electronic paper display, excellent in viewability compared with the liquid crystal display device. Most of the display devices are intended to realize bright display without using a polarization plate. However, such display devices realizes bright display of monochromatic color but having failed to realize color display with a brightness comparable to that of paper as yet because they have no choice but to use a color filter similarly as in the liquid crystal display device.
On the other hand, as a color liquid crystal display apparatus for effecting color display without using the color filter, a display apparatus using an electrically controlled birefringence (ECB)-type liquid crystal display device has been known. The ECB-type liquid crystal display device is constituted by a pair of substrates and liquid crystal sandwiched between the substrates, and is roughly classified into those of a transmission-type, in which each of the pair of substrates is provided outside with a polarization plate, and a reflection-type.
In the case of the ECB-type liquid crystal display device of the reflection-type, there are one-polarization plate type display device in which only one of the substrates is provided with a polarization plate and two-polarization plate type display device in which both of the substrates are provided with a polarization plate and a reflection plate is disposed outside each of the polarization plate.
In the case of the ECB-type liquid crystal display device of the transmission-type, linearly polarized light which comes in through one of the polarization plates is changed into elliptically polarized light consisting of respective wavelength light fluxes different in state of polarization by the action of birefringence of liquid crystal layer in a process of transmitting a liquid crystal cell. The elliptically polarized light enters the other polarization plate and the transmitted light having passed through the other polarization plate is colored light consisting of light fluxes of colors corresponding to light intensities of the respective wavelength light fluxes.
As described above, the ECB-type liquid crystal display device is capable of coloring light by utilizing the birefringence action of the liquid crystal layer of the liquid crystal cell and the polarization action of polarization plate, so that it causes no light absorption by the color filter, thus effecting bright color display at a high transmittance of light. In addition, the birefringence of the liquid crystal layer is changed depending on a voltage applied to the liquid crystal cell, so that by controlling the voltage applied to the liquid crystal cell, it is possible to change the color of the transmitted light or the reflected light. By utilizing this, it is possible to display a plurality of colors at one (the same) pixel.
FIG. 14 is a chromaticity diagram showing a relationship between an amount of birefringence (called retardation R) of the ECB-type liquid crystal display device and coordinates. From FIG. 14, it is found that the color at a retardation R from 0 to about 250 nm is achromatic color since the retardation range is located substantially at a center portion of the chromaticity diagram but is changed when the retardation exceeds the retardation range.
When a liquid crystal material having a negative dielectric anisotropy (−Δ∈) is used as the liquid crystal and liquid crystal molecules thereof are homeotropically (vertically) aligned with respect to the substrates, the liquid crystal molecules are inclined with voltage, so that an amount of birefringence is increased with a degree of the inclination of the liquid crystal molecules.
In this case, in a cross-nicol condition, the chromaticity is changed along a curve indicated in FIG. 14. More specifically, when the voltage is not applied, the retardation R is substantially zero, so that light does not pass through the display device to provide a dark (black) state. With an increase in voltage, brightness is increased in the order of black, gray, and white. When the voltage is further increased, the light is colored to change the color in the order of yellow, red, violet, blue, yellow, violet, light blue, and green.
As described above, under voltage application, the ECB-type liquid crystal display device is capable of changing the brightness between a maximum brightness and a minimum brightness in a modulation range on a low voltage side under and changing a plurality of hues.
Incidentally, as shown in FIG. 14, the color obtained by the change in retardation has a color purity which is considerably lower than those of colors having maximum purities which are located on an outer edge of the chromaticity distribution. As a method of compensating such a low purity, color filter is used in combination with the liquid crystal cell as described in Japanese Laid-Open Patent Application (JP-A) No. Hei 04-05265, whereby the color of ECB display is increased in purity by passing the light through a color filter of the same color. In the method described in JP-A Hei 04-052625, in order to obtain high-purity red, a pixel which is not used for blue display is provided with a red-type color filter or a yellow-type color filter to cut a red (short) wavelength component obtained by the ECB effect, thus providing high-purity red.
Hereinbelow, the range of retardation (0 to 250 nm) in which the brightness is changed in the order of black, gray, and white on the chromaticity diagram is referred to as a “brightness change range”, and a range of retardation (not less than 250 nm) in which chromatic color not less than yellow is changed is referred to as a “hue change range”. However, a border between the achromatic color and the chromatic color is not determined clearly, so that the border of 250 nm should be understood as a certain index thereof.
Incidentally, in the present invention, the color obtained by retardation (retardation change) is referred to but it means the color along the curve shown in FIG. 14. On the curve, three points at which the purity is maximum are located close to positions where the retardation is 450 nm, 600 nm and 1300 nm, and the color is visually recognized as red, green, and blue, respectively. However, before and after each of the three points, there is a 100 nm-range in which the color is substantially regarded as the corresponding color (red, green or blue), so that in the present invention, the colors in such ranges are also referred to as red, green and blue, respectively. Magenta is located at a point of 530 nm between the ranges of red and blue.
Generally, the color of color filter used in the liquid crystal display apparatus has a higher purity than that obtained by retardation, so that it is located outside the above described chromaticity distribution on the chromaticity diagram shown in FIG. 14. In the present invention, however, such color is also referred to as the same color.
Incidentally, the liquid crystal display apparatus (color display apparatus) using the conventional ECB-type liquid crystal display device was capable of effecting color display. However, this color display was based on a change in hue achieved by utilizing the birefringence effect, so that it was possible to control the hue by an amount of a voltage applied to the liquid crystal layer but it was difficult to obtain a high-purity display color. Further, for displaying green, the color display apparatus had no choice but to use a high retardation value of 1300 nm. In addition, it was also difficult to display smooth gradation color at the hue.
Accordingly, the color display was only effected with the limited number of display colors.